Hello MacEachaidh
One other option is to keep on doing it into epub and then convert to mobi with calibre.
Another is to create an html file from InDesign (if it does create those) and use mobipocket creator to make your mobi (*.prc) eBook.
For this one you can check, per example, my today’s similar post about how to do it in:
https://www.mobileread.com/forums/sho...68&postcount=2
Best regards,